From: Wanpeng Li <wanpen...@tencent.com>
vCPUs are very unlikely to get preempted when they are the only task
running on a CPU. PV TLB flush is slower that the native flush in that
case. In addition, avoid traversing all the cpus for pv tlb flush when
steal time is disabled since pv tlb flush depends on the field in steal
time for shared data.